boost::static_visitor with multiple arguments

Question

typedef boost::variant<int, double> Type;
class Append: public boost::static_visitor<>
{
public:
    void operator()(int)
    {}

    void operator()(double)
    {}

};

Type type(1.2);
Visitor visitor;
boost::apply_visitor(visitor, type);

Is it possible to change the visitor such that it receives extra data as follows:

class Append: public boost::static_visitor<>
{
public:
    void operator()(int, const std::string&)
    {}

    void operator()(double, const std::string&)
    {}
};

This string value changes during the lifetime of the Append object. Passing the string in via the constructor is not an option in this case.

Answer 1

The "additional argument" that gets given to each call is the this pointer. Use it to pass whatever additional information that you need:

#include <boost/variant.hpp>
typedef boost::variant<int, double> Type;
class Append: public boost::static_visitor<>
{
public:
    void operator()(int)
    {}

    void operator()(double)
    {}
    std::string argument;
};

int main() {
    Type type(1.2);
    Append visitor;
    visitor.argument = "first value";
    boost::apply_visitor(visitor, type);
    visitor.argument = "new value";
    boost::apply_visitor(visitor, type);
}

Answer 2

Another option is to bind the extra arguments. You visitor class could look like this:

class Append: public boost::static_visitor<>
{
public:
    void operator()(const std::string&, int)
    {}

    void operator()(const std::string&, double)
    {}
};

Call it like so:

std::string myString = "foo";
double value = 1.2;
auto visitor = std::bind( Append(), myString, std::placeholders::_1 );
boost::apply_visitor( visitor, value );